High speed serial differential interfaces depend on extremely low output jitter to maintain error-free (or low bit-error rate) communication. Common mode voltage fluctuation of the output differential signals also presents a problem since receiver circuitry requires a stable, common mode input voltage for optimal performance. Conventionally, current mode differential output drivers are limited in architecture, generally consisting of a simple current source and switches which channel current to either a data input line or a complementary data input line.
Differential output drivers generally work in conjunction with predriver circuitry. The predriver is generally responsible for opening and closing the switch devices of the output drivers. Generally, the output signals from the predriver, which open/close the switches of the line driver, must maintain a crossover point that is both stable in voltage and time. Any temporal variation of the crossover point would translate directly into line driver output jitter.
Conventionally, line driver output jitter is primarily caused by sensitivity to voltage (Vcc) fluctuation which speeds up or slows down the transition of the predriver signals. Voltage variation of the crossover point will have a lesser effect on output jitter; but it will produce undesirable common mode variation during the transition of the output signals. Conventional ground reference differential line drivers generally use P-channel metal oxide semiconductors (PMOS) switches to pull-up node voltages to drive differential output signals.